1. Field of the Invention
This invention concerns a technique of producing molten iron by arc heating of pre-reducing iron. More specifically, it relates to a technique of supplying pre-reducing iron to a stationary non-tilting type melting furnace and melting the iron by arc heating mainly comprising radiation heating, in which molten iron at stable quality is produced at a high efficiency while improving the life of refractory in the melting furnace.
2. Description of the Related Art
As a method of producing liquid iron (molten iron) by heating solid iron, a technique of charging solid iron into a melting furnace such as an electric furnace and melting them by arc as a heating source has been known so far. Further, direct reduced iron has been used as the solid iron in recent years.
Reduced iron is produced basically by reducing iron oxide sources such as iron ores and various methods have been proposed so far for producing reduced iron. For example, direct iron making process of producing reduced iron by directly reducing iron oxide sources such as iron ores or iron oxide pellets by reducing agents such as carbon materials or reducing gases have been known. A shaft furnace process, an SL/RN process or the like can be listed as an example of the direct iron making process. The shaft furnace process can include a Midrex process as a typical example. In this process, an iron oxide source in a furnace is reduced by blowing a reducing gas produced, for example, from a natural gas through a tuyere disposed at a lower portion of the shaft furnace, which is a technique of reducing the iron oxide source by utilizing the reducing gas. In the SL/RN process, carbon material such as coal is used as the reducing agent and the carbon material is heated together with the iron oxide source such as iron ores by a heating means such as a rotary kiln to reduce the iron oxide source. In addition, as the direct iron making process other than those descried above, U.S. Pat. No. 3,443,931 describes, for example, a method of mixing a carbon material and iron oxide fines into compacts and heating them on a hearth to reduce the iron oxide.
Further, it has also been known a method of mixing a carbon material and iron oxide fines into compacts, reducing them under heating on a rotary hearth and further melting and separating the resultant reduced iron into a slag component and a metallic iron component to produce a high purity metallic iron as disclosed, for example, in U.S. Pat. No. 6,036,744, Japanese Patent Laid-open Application No. Hei 9-256017, Japanese Patent Laid-open Application No. Hei 12-144224. Direct reduced iron produced by reducing iron oxide sources as described above are frequently used in the technique of producing molten iron.
An electric furnace and a submerged arc furnace can be shown as examples of the melting furnace for melting direct reduced iron. For example, in a tilting type melting furnace, a furnace body has to be tilted upon discharge of molten iron in which a batch treatment is conducted. In a case of transporting direct reduced iron produced continuously in a reduced iron production plant directly to a melting furnace where solid direct reduced iron is melted, continuous processing can not be conducted by a single tilting type melting furnace and it is not preferred with a view point of ensuring operation at high productivity. If several tilting type melting furnaces are used and direct reduced iron is supplied continuously to them, it is possible to continuously melt direct reduced iron. However, the scale of the facility has to be enlarged for installing several tilting type melting furnaces. In addition, since the tilting device for tilting the furnace has a complicate structure, it increases the construction cost, as well as operation cost and maintenance cost for operating several furnaces.
Further, in a case of the tilting type melting furnace, relatively small sized furnaces are used with a view point of the scale of the facility and the construction cost, because the size of the tilting device for the furnace is increased when the furnace with a large inner diameter is used. However, when direct reduced iron is melted by a small-sized tilting type melting furnace, furnace wall refractories in contact with molten slags suffer from erosion by arc radiation, and periodical repairing is necessary to the refractories, and the operation has to be interrupted.
Further, direct reduced iron supplied contains slag component such as SiO, Al2O3 and CaO derived from gangue in the iron ores used as the raw material and ashes in the carbon material, and the composition of them and the reduction rate vary with time depending on the fluctuation of operation conditions in the reducing furnace and the like.
Accordingly, when the direct reduced iron is melted by a small sized tilting type melting furnace, it results in a problem that the composition of the molten iron produced are different on every batch. Further, for overcoming the difference in the composition of the molten iron on every batch as described above, the molten iron is discharged after controlling the composition in the furnace. However, an excess electric energy is required for preventing lowering of molten iron temperature during such control for the composition. In addition, since the control for the composition is conducted in the furnace, operation time required per batch increases to inevitably lower the productivity. As described above, when the tilting type melting furnace is used, there are various problems in ensuring operation at high productivity.
Further, in a case of melting direct reduced iron at, for example, a submerged arc furnace, top ends of electrodes are submerged in a slag layer as shown in FIG. 4 and electric current is supplied, to generate Joule heat among the solid reduced iron in the slag layer or on the slag layer to melt the iron. However, since the resistance lowers as the metallization of the reduced iron to be melted is higher, the energy consumption for melting the direct reduced iron has to be increased, which results in lowering the productivity. Particularly, when the solid reduced iron is fed not uniformly in the furnace, the surface of the slag layer is overheated to cause an accident of leaking molten iron or molten slag from the furnace, so that careful operations have been required for the feeding of the solid reduced iron.
In the submerged arc furnace, while the direct reduced iron can be fed continuously since molten iron can be discharged properly from the bottom of the furnace, the productivity for the molten iron is low as described above. Accordingly, in existing submerged arc furnaces, the scale of the construction per unit production of molten iron is increased such as by the use of a large sized furnace for ensuring production amount, but since the use of the large sized furnace increases the electric power consumption and construction cost, the productivity has not yet been improved.
This invention has been accomplished in view of the foregoing problems and it intends to provide a technique, for producing a molten iron by arc heating a pre-reducing iron in a melting furnace, capable of withstanding erosion to furnace wall refractory in a melting furnace to improve the working life and capable of producing a molten iron with a homogenized composition while keeping high productivity.
The technique of the present invention capable of solving the foregoing subject is a method for producing a molten iron comprising feeding a pre-reducing iron to a stationary non-tilting type melting furnace and melting the iron by arc heating mainly composed of radiation heating, the melting being performed while keeping a refractory wearing index RF represented by the following equation at 400 MWV/m2 or less.
RF=Pxc3x97E/L2
[wherein RF represents a refractory wearing index (MWV/m2); P represents an arc power for 1 phase (MW); E presents an arc voltage (V); and L represents the shortest distance (m) between the electrode side surface of the tip within an arc heating type melting furnace and the furnace wall inner surface.]
Further, the present invention provides a stationary non-tilting arc heating type melting furnace for melting a pre-reducing iron by arc heating mainly composed of radiation heating, the melting furnace having a pre-reducing iron feeding mechanism, electrodes for arc heating and a molten iron discharging mechanism, the melting being performed while keeping a refractory wearing index RF represented by the following equation at 400 MWV/m2 or less.
RF=Pxc3x97E/L2
[wherein RF represents a refractory wearing index (MWV/m2); P represents an arc power for 1 phase (MW); E presents an arc voltage (V) and L represents the shortest distance (m) between the electrode side surface of the tip within an arc heating type melting furnace and the furnace wall inner surface.]
L=ID/2xe2x88x92PCD/2xe2x88x92DE/2
[wherein ID represents the inside diameter (m) of the melting furnace; PCD represents an electrode pitch circle diameter (m); and DE represents an electrode diameter (m).]